1. Field of the Invention
This invention generally relates to a non-coherent frequency shift keying transmitter, and more particularly to a non-coherent frequency shift keying transmitter using a digital interpolation synthesizer.
2. Description of Related Art
Digital wireless communication system, such as mobile phone communication system using bluetooth protocol, has been widely used by the consumers and becomes an indispensable tool for a modern daily life. In a digital wireless communication system, because of the high efficiency requirement for transmission, the non-coherent frequency shift keying (FSK) technology plays an important role in generating RF signals for carrying the transmitting signals.
In a digital wireless communication system, a non-coherent FSK modulator includes a micro processing unit (MPU) and a phase locked loop (PLL). The non-coherent FSK modulator converts the signals from the baseband frequency to radio frequency (RF). FIG. 1 is a block diagram of a conventional non-coherent FSK modulator 100. Referring to FIG. 1, the MPU 102 receives the source signal; the source signal is up-converted by the PLL (104, 106, and 108), processed by the power amplifier, and then transmitted. The PLL includes a phase frequency detector 104, a loop filter 106, and a voltage controlled oscillator (VCO) 108 as shown in FIG. 1.
However, the conventional non-coherent FSK modulator has the following disadvantages. First, the PLL requires a relatively long period for settling time to transmit a precise frequency and thus it is power consuming. Further, the quality of the signal transmitted by the modulator is neither satisfying. An analog circuit designer has to trade off between signal stability and short lock time, yet usually neither condition is met. Further, the modulator suffers from fabrication migration thus transmission quality is degraded for limitation of PLL bandwidth.